Campaigners who want Britain to leave the European Union say that if the country chooses to stay in the bloc, it would one day face a wave of immigration from Turkey.

The official ‘Vote Leave’ campaign claims Turkey is on course to join the EU, and says that would lead to a million Turkish citizens coming to Britain, including, in their words, "murderers, terrorists and kidnappers."

The claims, made Sunday by junior Armed Forces Minister Penny Mordaunt, drew a sharp rebuke from Prime Minister David Cameron.

"Turkey joining the EU is not remotely in the cards,” he said in a television interview Sunday. “At the current rate of progress it would be decades, literally decades, before this even had a prospect of happening and, even at that stage, we would be able to say 'No.'"

Existing EU members effectively have a double veto on whether Turkey joins, says Damian Chalmers, professor of EU law at the London School of Economics.

“There has to be unanimous agreement in the Council of Ministers where the national governments sit. Then after that, Turkey would have to sign a treaty with each of the existing 28 member states to join the EU. Now at that moment, a national parliament could veto it."

The debate comes at an awkward time. Turkey agreed in March to take back refugees from Europe, partly in return for the opening up of new chapters in

Ankara’s EU accession process. Turkish ministers have demanded clarity from Brussels, but Chalmers says that in reality, the negotiations are occurring at a glacial pace.

“Turkey has not negotiated successfully 35 policy areas with the EU before any treaty can be signed. It has been negotiating since 2005. So far, only one of those 35 has been successfully concluded.”

Britain’s vote on June 23 could have a much wider impact on Europe’s relationship with Turkey, says Professor Ibrahim Sirkeci of Regent's University in London.

“The referendum in the UK will generate a wave that will affect Europe substantially, especially if it is in favor of leaving the EU,” he said.

With polls showing the ‘Remain’ camp pulling into the lead, ‘Vote Leave’ campaigners are focusing on fears over immigration. Launching a so-called ‘battle bus’ to tour the country last week, the leader of the UK Independence Party, Nigel Farage, sought to put sovereignty at the heart of the referendum.

“We want our borders back. We want our passports back. We want our country back. And if everyone that agrees with us goes out to vote on June 23, we will make it UK Independence Day.”

Meanwhile, Prime Minister Cameron warned Monday that a vote to leave the EU would be in his words, the "self-destruct option," pushing Britain into recession and causing half a million job losses.